Using different tubes than what amp was designed for

leib10

Active member
I've heard mixed things on this- that it can lead to transformer damage over time, etc. Can anyone shed some light on this? Thanks!
 

Findthetone1

Well-known member
It's kinda like putting spark plugs in your engine that weren't designed for it. Generally considered a bad idea.
 

FourT6and2

Well-known member
Depending on the amp's design and the transformers, you could do it but it usually requires a few resistor changes if the amp wasn't designed to run different types of tubes.
 

HilltopExplosion

New member
HEY AMP GURU'S!! If I tell him something wrong, correct me.

From what I understand you can't use and EL34 in a 6L6 amp. It would take mods beyond what I can tell ya. But you can run 6L6 or 6L6 family in a EL34 amp if pins 8 and 1 are bridged on the power tube socket. This will allow you to run it only if the bias range is ok. I put TAD 6L6GC STR's in my BE 100. When I did, I lowered the bias adjustment as low as it would go. I think the EL34's were around 15ma. When I put the 6l6's in, the DC voltage was the same across pins 8 and 3 as EL34, but the bias was almost 40ma! So instead of doing a resistor change (the one going to the bias adjuster) I lowered the voltage using a variac until I found a balance between AC voltage DC voltage and the tube bias, which I settled on 65%. I only did this after I asked if it was OK on here and the Friedman board. Clear as mud right? :LOL: :LOL:
 

Loudness250

New member
Is there a specific tube that you are wanting to try in a certain amp? That would probably be easier to answer. I put kt88's in a 2203 with some slight changes it was fine, didn't damage anything. I had a kt66 amp and I put el34's in it with no mods and it was fine. My friend had a tech mod his 5150II to run el34's, that was a little wierd, it wasn't so reliable.
 

leib10

Active member
maddnotez":1qbqpl5d said:
Within the same bias so like a super cold KT88 set in an EL34
Amp.

This. I've been running KT88s in my Uberschall Rev. 2 (which normally takes EL34s) and was wondering if that was going to cause any damage. I have a bias mod on this amp, so it biases up just fine. I was just wondering if that would put additional stress on the transformer or any other part.
 

psychodave

Well-known member
I use 6550's, 6L6's and EL34's in my Marshall modded amps without changing anything but the bias resistors so they can be properly biased. That's it. Nothing else. These are amps, not highly sophisticated NASA equipment. lol

Look at the boogie Rectos. You can change between EL34's and 6L6's. The simple switch is only a bias resistor.
 

Shawn Lutz

New member
It really is a big not a big deal if biased and resistors changed. I prefer myself to stay with what the amp was designed for, including tubes, cabs and speaker types ;)
 

PatF

New member
The transformer specs do matter. The input impedance of the output transformer may not be a good match for a different type of tube. The power transformer winding ratings both B+ and heater should be considered. Putting KT-88's in a 6L6 amp will add a few amps to the the heater demand and can burn up a transformer.
 

jc986

New member
leib10":ef57v0ql said:
maddnotez":ef57v0ql said:
Within the same bias so like a super cold KT88 set in an EL34
Amp.

This. I've been running KT88s in my Uberschall Rev. 2 (which normally takes EL34s) and was wondering if that was going to cause any damage. I have a bias mod on this amp, so it biases up just fine. I was just wondering if that would put additional stress on the transformer or any other part.

I'd just shoot Bogner an email and ask. I asked about putting KT88's in my Rev Blue and they said it wouldn't be a problem at all. I was even able to order a cool enough grade of KT88's that they bias up just fine with the stock bias range.
 

fusedbrain

Well-known member
My understanding of this is similar to Hilltop's: Assuming that the bias circuit has been modded, you will generally be ok with any of EL34's,6CA7's, KT88, 6550, KT66, KT77 & 6L6GC in any amp designed for any of the above, with the exception of an amp designed specificly for 6L6's or KT66's.
In a 6L6 amp, as Hilltop said, pins 1 & 8 are tied together and would need to be separated, but the bigger issue for 6L6 amps running the other tubes is the heater current draw. 6L6's draw considerably less heater current than all the other tube types, and if the power transformer is designed ONLY for 6L6's and can't handle the increased heater current draw, your gonna have problems.
The issue with KT66's is the plate to plate impedance, where an amp designed around KT66's would have an output transformer with a primary impedance of around 8k/pair of tubes, where all the other tube types would be looking for approx 3.4 - 4k primary impedance / pair. You can usually get around this by mis-matching your cab to the speaker output on the amp, so running KT66's in an EL34 designed amp, or vice versa, is not usually a big deal (other than biasing)

One other thing to keep in mind when switching to the big bottle 6550's or KT88's is the plate dissipation wattage you use in your bias calculation. For example, 6550's are rated @ 35watts, and most KT88's are 42 watts. Biasing these up to 60-70 of their rated power is going to put more of a load on the power transformer than if you use the 25 watt figure for EL34's.
Bogner says it's no problem with their Rev Blue to run KT88's properly biased, and their power transformers are likely very high quality,
But... be aware that with some amps, this MAY be an issue. For example, there are stories out there where the Marshall 2203KK is designed to be biased as a 25watt per tube amp, and if you crank up the bias on the the 4 KT88's to dissipate 42 watts per tube, the power transformer Marshall uses in these amps can't take the load at high volumes and can overheat and fail.
Stay away from doing 2 hour sets in stadiums and you'll probably be OK, but YMMV.
 

leib10

Active member
Thanks for the reply. I bias the KT88s at 38mA, which is fairly cool and shouldn't place too much stress on the amp.
 

FourT6and2

Well-known member
There is an output impedance change as well, when switching tubes. For example, the SLO was designed for 6L6s. I'm talking about the output transformer. So even if you mod the bias section of an amp designed around one specific tube type, you should make sure the impedance change isn't too drastic. This isn't the case for every amp, though. It's design-specific.
 

leib10

Active member
Talked to Charley at Bogner, and he said that KT88s in the Rev 2 are just fine if they bias up well.
 

maddnotez

Well-known member
leib10":9achivp2 said:
Talked to Charley at Bogner, and he said that KT88s in the Rev 2 are just fine if they bias up well.

What are some of the cooler KT88's out there?

Was going back and forth between EH6CA7's and JJE34L's but I suppose the KT88's could solve the issue :p.

I'm also wondering if it's worth getting the mod to just run a hotter set vs having to have a cooler set.
 

jc986

New member
I just request the coolest grade of JJ KT88's from Eurotubes and they have plenty of room to go hotter or cooler to taste when biased to spec. Bogner recommended 35ma per tube.
 
Top